The present invention relates to a piezoelectric actuator and, in particular, to a stacked type piezoelectric activator or a laminated type.
Recently, the piezoelectric actuator has been used as a micro-displacement control element, a pulse driven motor, a ultra-sonic motor and the like in various applications such as servo-control systems in optical systems, micro-machining systems, precision-machining systems, automobiles (brakes, engine valves, suspensions, and others) and the like.
A stacked-type piezoelectric actuator comprises a generally rectangular stack body or laminate body having opposite first and second outer side surfaces and first and second external electrodes formed on the first and second outer side surfaces, respectively. The stack body comprises a plurality of rectangular piezoelectric ceramic plates or layers and a plurality of internal electrode layers which are alternately stacked or laminated on one another in a first or stacking direction to form the stack body. Alternate ones of the internal electrode layers in the stacked direction are connected to the first external electrode, while the remaining alternate ones of the internal electrode layers are connected to the second external electrode.
In use, an electric voltage is applied across the external electrodes. Adjacent internal electrode layers sandwiching a particular one of the piezoelectric ceramic layers therebetween serve as mutually counter electrodes for applying an electric field to the particular piezoelectric ceramic layer. Thus, the particular piezoelectric ceramic layer, and therefore the stack body, is displaced in response to the application of the electric voltage across the external electrodes. It is necessary to insure electrical insulation between the adjacent internal electrode layers, while the adjacent internal electrode layers are connected with the first and second external electrodes, respectively.
To this end, a first conventional stacked-type piezoelectric actuator typically has an internal electrode structure where alternate internal electrode layers extend in the stack body and are only exposed at the first outer side surface of the stack body and are connected to the first external electrode on the first outer side surface. The remaining alternate internal electrode layers also extend in the stack body and are exposed at the second outer side surface of the stack body and are connected to the second external electrode on the second outer side surface.
In the first conventional stacked-type piezoelectric actuator, it is possible to insure electrical insulation between the adjacent internal electrode layers, while the adjacent internal electrode layers are connected with the first and second external electrodes, respectively. Further, it is possible to accept a smaller interval between the adjacent internal electrode layers in the stack body. That is, it is possible to reduce a thickness of each of the piezoelectric ceramic layers.
However, each of the piezoelectric ceramic layers has a C-shape stripe area uncovered with any one of the internal electrode layers formed on its surface. The C-shape stripe extends along three outer side surfaces except the first or the second outer side surface of the stack body. Therefore, the C-shape stripe portion of each of the piezoelectric ceramic layers is not applied with the electric field even when the electric voltage is applied across the external electrodes. Accordingly, the stack body has portions which are not displaced adjacent opposite side surfaces other than the first and second outer side surfaces of the stack body.
Therefore, stress is concentrated at peripheral portions of the internal electrode layers by displacement of the stack body caused in response to the application of the electric voltage thereto. As a result, the displacement of the body is restrained and any mechanical damage often occurs at the stress-concentrated portions.
In order to resolve the problem, a second conventional stacked-type piezoelectric actuator has a different internal electrode structure. In the structure, all of the internal electrode layers extend in the stack body and are exposed in all of the outer side surfaces of the stack body including the first and second outer side surfaces. Each of the internal electrode layers covers the entire surface of the adjacent piezoelectric ceramic layers. Therefore, each of the piezoelectric ceramic layers and therefore the stack body is displaced at the entirety when the electric voltage is applied to the stack body.
However, the stack body is required to have protective insulator films on the first and the second outer surfaces so as to electrically insulate the alternate internal electrode layers from the second external electrode formed on the second outer surface and also electrically insulate the remaining alternate internal electrode layers from the first external electrode formed on the first outer side surface. This results in a problem that a thin type actuator is difficult to produce. This is because it is impossible to form the protective insulator films in the case where an interval between adjacent internal electrode layers is less than 60 .mu.m.